The field of this invention relates to sound absorbers and more particularly to a sound absorber which is composed of a plurality of parallel, terminated, acoustical waveguides which are grouped into a bundle and located within a structure mounted within a gas passage duct.
In many ducts, such as air conditioning ducts, gas turbine inlet or exhaust ducts, and the like, it is desirable to attenuate the sound transmitted in the duct while at the same time permitting flow passage with a minimum back pressure. In the past, it has been customary to line the interior wall of the ducts with acoustical material to absorb unwanted sound or to locate a sound attenuating structure along the longitudinal center axis of the duct. Previous sound absorbing structures are capable of efficiently absorbing the sound, so long as they are dry and clean, but fail to absorb sound if dirty or wet and hence are often unsatisfactory.
Gas flow within ducts or passages frequently include particles of dust and dirt and/or small particles of moisture or other liquid. Previously, the most common types of sound absorbing material have been some form of fiberglass matting, loosely packed fibrous material, or the like. Thus a sound attenuator, when constantly subjected to a gas which contains particles of dirt, dust, or liquids becomes clogged and the sound absorption effectiveness is significantly decreased. After a certain period of time, the effectiveness has been diminished to such an extent that it is necessary to clean or replace the attenuator.
Previously, there have been other types of sound absorbers which are not constructed in the form of a mat, foam, or a loosely packed material, but which take the form of a solid cellular structure. An example of such a structure is shown in U.S. Pat. No. 3,913,702, issued Oct. 21, 1975, by Leslie S. Wirt and Duane L. Morrow, entitled CELLULAR SOUND ABSORPTIVE STRUCTURE. The sound absorptive structure of the aforesaid patent is composed of a bundle of parallel, terminated acoustical waveguides with the inner end of the waveguides terminating upon an oblique planar member. A sound wave arriving at the open end of the bundle of waveguides will encounter at least one waveguide which is effectively resonant at the frequency of the arriving wave. Therefore, there is always a waveguide available to absorb sound over a wide range of sound frequencies. A structure of this type has been found to be quite efficient at absorbing sound. However, there has been one problem in that in the fixedly mounting of the aforesaid patented sound absorber, within a gas passage which contains minute material particles, it will be found that over a period of time, the absorber becomes clogged and its effectiveness is diminished. If the open ends of the waveguides are disposed above the closed ends, they cannot dump or drain the dirt or liquids.